Absorption spectroscopy, and more particularly, laser absorption spectroscopy (LAS) is a well-known laser-based technique used for detecting and monitoring constituents in a medium, (gas, fluid, or solid). Absorption spectroscopy, in general, measures the absorption of radiation due to its interaction with a sample medium. In most cases, the absorption of one media depends upon the media constituents and their concentration. Each constituent's absorption is uniquely wavelength dependent and determined by its atomic or molecular structure and mass. Using narrow linewidth tunable laser sources, one can tune to a constituent specific absorption wavelength and determine the respective concentration present in the probed sample volume passed through by the radiation. The detection sensitivity afforded by LAS systems is typically limited by the performance of the laser source, detection techniques, constituent absorption strength and optical and electronic noise sources. A method used to enhance the signal-to-noise ratio of the LAS system is to increase the effective path length through which radiation propagates (Governed by Beer-Lambert law). For open path measurements of gaseous constituents, one can increase the physical distance between the optical source (i.e. laser) and the optical detector (typically up to several hundred meters). For in-situ sampling, this simple approach is not feasible both in terms of loading the cell with a sample for several reasons. The gas would need to be pumped through such cell and would significantly increase in the space required for storage and operation of the device.
Another approach to increase the optical path length is to provide a folded path in the form of a cell enclosing a small volume into which the media is introduced. A laser beam is introduced into such a cell composed of two opposing mirrors, in which the light is reflected back and forth multiple times through the sample cell prior to leaving the cell and reaching an optical detector. This is the basic concept provided by a so-called “Herriott Cell.” The basic Herriott Cell provides two spherical end mirrors that are spaced apart from one another. A light (generally a laser) beam is introduced, typically off-axis, through a non-refracting aperture in one of the mirrors. The light is reflected back and forth between the two mirrors creating a distinct pattern. The shape of the pattern can be determined based on the physical distance between the two mirrors, the radius of curvature of the mirrors, the angle and position the light beam is injected into the cavity. As the light is reflected between the two mirrors, an effective path length that is much longer than the physical distance between the two mirrors is provided. As a result, the beams' absorption increases proportional with the distance it propagates through the sample and hence the signal-to-noise ratio of the detected light can be significantly improved.
The present invention provides an improved optical multi-pass cell with a smaller sampling volume and means to re-launch the beam into the cell after it completes a predefined propagation pattern, hence further increasing the effective length of the light path between the two end mirrors. The relay mirrors may be positioned between the two end mirrors or outside of two end mirrors. The optical source and optical detector may be positioned between the two end mirrors or outside of the two end mirrors. Further, in some embodiments, the optical multi-pass cell provides a rod or other support member between the two end mirrors. The rod can support the two end mirrors and in some embodiments house the laser source and detector. The rod can also decrease the effective sample volume between the end mirrors.